The present invention relates to a code division multiple access mobile communication system and its transmission power control method. More particularly, the present invention relates to a packet communication system and its transmission power control method using reservation based access control.
In a CDMA method, a plurality of mobile terminals share the same frequency band to communicate with a single base station. Therefore, for example, if mobile terminals A and B transmit modulated signal waves to the base station, the signal (not desired to be received) transmitted by the mobile terminal B interferes with the signal (desired to be received) transmitted by the mobile terminal A, and the communication of the mobile terminal A with the base station is obstructed. The degree of interference depends on the received level of a signal (not desired to be received) at the base station. If the degree of interference becomes large to some level or more, communication between the mobile terminal and base station becomes impossible.
If the transmission power of each mobile terminal can be controlled to always limit the signal level received at the base station to a minimum necessary reception power, it becomes possible to maximize the number of channels capable of being communicated by the base station. The more the transmission power shifts from the minimum necessary reception power, the less the number of channels capable of being communicated by the base station.
As transmission power control techniques of CDMA mobile communication, an IS-95 transmission power control method is known described in TIA/EIA/IS-95 which is a standard system of digital cellar phones adopted in North America. The IS-95 transmission power control method will be described in the following.
Since two way communication is essential for cellular phones, a pair of an uplink traffic channel and a downlink traffic channel is used for the communication between the base station and a mobile terminal. The uplink traffic channel is a channel for transmitting data from a mobile terminal to the base station, and a downlink traffic channel is a channel for transmitting data from the base station to the mobile terminal.
The base station measures the reception power of data transmitted from each mobile terminal and generates a transmission power control signal in accordance with the measured reception power. If the reception power of data is larger than a target reception power, the base station generates a transmission power control signal “1” for this mobile station. Conversely if the reception power of data is smaller than the target reception power, the base station generates a transmission power control signal “0” for this mobile station. The generated transmission power control signal is inserted into data to be transmitted from the base station to a mobile terminal, and the transmission data with the transmission power control signal is transmitted to the mobile terminal. The mobile terminal controls to reduce the transmission power if the received transmission power control signal is “1”, and to increase it if “0”.
This transmission power control will be described specifically with reference to FIG. 12. Each mobile terminal 1 to n and the base station communicate with each other by using a pair of an uplink traffic channel and a downlink traffic channel. The upper row of each pair represents transmission data of the downlink traffic channel, and the lower row represents transmission data of the uplink traffic channel. The width of transmission data, particularly uplink transmission data, is drawn to correspond to a reception power of the uplink data at the base station.
When the base station communicates with the mobile terminal 1, it inserts transmission power control signals 132a, 132b, 132c, . . . into a downlink traffic channel 130a to the mobile terminal 1. The mobile terminal 1 changes its transmission power of the uplink transmission data in accordance with the transmission power control signal obtained from the received channel 130a. As above, the transmission power control of the mobile terminal 1 is performed by using the downlink traffic channel 130a. Similar transmission power control is performed also for other mobile terminals 2 to n.